International OPEN ACCESS Journal Of Modern Engineering Research (IJMER) | IJMER | ISSN: 2249–6645 | www.ijmer.com | Vol. 4 | Iss.7| July. 2014 | 30| Strengthening Of RC Beam Using FRP Sheet E. Rakesh Reddy ABBREVIATIONS ACI American Concrete Institute CFRP Carbon Fibre Reinforced Polymer BM Bending Moment EB Externally Bonded FEM Finite Element Method FRP Fibre Reinforced Polymer FRPC Fibre Reinforced Polymer Composite GFRP Glass Fibre Reinforced plastic HSC High Strength Concrete HYSD High Yield Strength Deformed IC Intermediate Crack IS Indian Standards NSM Near Surface mounted PSC Portland Slag Cement RC Reinforced Concrete RHSC Reinforced High Strength Concrete NOTATIONS D Overall Depth of the Beam B Breadth of the Beam d Effective Depth L Span Length of the Beam fck Characteristic Cube Compressive Strength of Concrete fy Tensile Strength of the Bar Abstract: Strengthening structures via external bonding of advanced fibre reinforced polymer (FRP) composite is becoming very popular worldwide during the past decade because it provides a more economical and technically superior alternative to the traditional techniques in many situations as it offers high strength, low weight, corrosion resistance, high fatigue resistance, easy and rapid installation and minimal change in structural geometry. Although many in-situ RC beams are continuous in construction, there has been very limited research work in the area of FRP strengthening of continuous beams. In the present study an experimental investigation is carried out to study the behavior of continuous RC beams under static loading. The beams are strengthened with externally bonded glass fibre reinforced polymer (GFRP) sheets. Different scheme of strengthening have been employed. The program consists of fourteen continuous (two-span) beams with overall dimensions equal to (150×200×2300) mm. The beams are grouped into two series labeled S1 and S2 and each series have different percentage of steel reinforcement. One beam from each series (S1 and S2) was not strengthened and was considered as a control beam, whereas all other beams from both the series were strengthened in various patterns with externally bonded GFRP sheets. The present study examines the responses of RC continuous beams, in terms of failure modes, enhancement of load capacity and load deflection analysis. The results indicate that the flexural strength of RC beams can be significantly increased by gluing GFRP sheets to the tension face. In addition, the epoxy bonded sheets improved the cracking behaviour of the beams by delaying the formation of visible cracks and reducing crack widths at higher load levels. The experimental results were validated by using finite element method. Keywords: continuous beam; flexural strengthening; GFRP; premature failure; debonding failure.
Strengthening structures via external bonding of advanced fibre reinforced polymer (FRP) composite is becoming very popular worldwide during the past decade because it provides a more economical and technically superior alternative to the traditional techniques in many situations as it offers high strength, low weight, corrosion resistance, high fatigue resistance, easy and rapid installation and minimal change in structural geometry. Although many in-situ RC beams are continuous in construction, there has been very limited research work in the area of FRP strengthening of continuous beams. In the present study an experimental investigation is carried out to study the behavior of continuous RC beams under static loading. The beams are strengthened with externally bonded glass fibre reinforced polymer (GFRP) sheets. Different scheme of strengthening have been employed. The program consists of fourteen continuous (two-span) beams with overall dimensions equal to (150×200×2300) mm. The beams are grouped into two series labeled S1 and S2 and each series have different percentage of steel reinforcement. One beam from each series (S1 and S2) was not strengthened and was considered as a control beam, whereas all other beams from both the series were strengthened in various patterns with externally bonded GFRP sheets. The present study examines the responses of RC continuous beams, in terms of failure modes, enhancement of load capacity and load deflection analysis. The results indicate that the flexural strength of RC beams can be significantly increased by gluing GFRP sheets to the tension face. In addition, the epoxy bonded sheets improved the cracking behaviour of the beams by delaying the formation of visible cracks and reducing crack widths at higher load levels. The experimental results were validated by using finite element method
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International
OPEN ACCESS Journal Of Modern Engineering Research (IJMER)
fck Characteristic Cube Compressive Strength of Concrete
fy Tensile Strength of the Bar
Abstract: Strengthening structures via external bonding of advanced fibre reinforced polymer (FRP)
composite is becoming very popular worldwide during the past decade because it provides a more
economical and technically superior alternative to the traditional techniques in many situations as it
offers high strength, low weight, corrosion resistance, high fatigue resistance, easy and rapid installation and minimal change in structural geometry. Although many in-situ RC beams are
continuous in construction, there has been very limited research work in the area of FRP
strengthening of continuous beams. In the present study an experimental investigation is
carried out to study the behavior of continuous RC beams under static loading. The beams are
strengthened with externally bonded glass fibre reinforced polymer (GFRP) sheets. Different scheme
of strengthening have been employed. The program consists of fourteen continuous (two-span) beams
with overall dimensions equal to (150×200×2300) mm. The beams are grouped into two series
labeled S1 and S2 and each series have different percentage of steel reinforcement. One beam from
each series (S1 and S2) was not strengthened and was considered as a control beam, whereas all
other beams from both the series were strengthened in various patterns with externally bonded GFRP
sheets. The present study examines the responses of RC continuous beams, in terms of failure modes,
enhancement of load capacity and load deflection analysis. The results indicate that the flexural strength of RC beams can be significantly increased by gluing GFRP sheets to the tension face. In
addition, the epoxy bonded sheets improved the cracking behaviour of the beams by delaying the
formation of visible cracks and reducing crack widths at higher load levels. The experimental results
λ Load Enhancement Ratio φ Diameter of the Reinforcement M Moment of Resistance
E Modulus of Elasticity
I Moment of Inertia
F Global Nodal Force Vector K Stiffness Matrix
U Global Nodal Displacement Vector
u Displacement Field
Ni Interpolation Function
ui Nodal Displacements
I. Introduction 1.1 General
A structure is designed for a specific period and depending on the nature of the structure, its design life varies. For a domestic building, this design life could be as low as twenty-five years, whereas for a public
building, it could be fifty years. Deterioration in concrete structures is a major challenge faced by the
infrastructure and bridge industries worldwide. The deterioration can be mainly due to environmental effects,
which includes corrosion of steel, gradual loss of strength with ageing, repeated high intensity loading, variation
in temperature, freeze-thaw cycles, contact with chemicals and saline water and exposure to ultra-violet
radiations. As complete replacement or reconstruction of the structure will be cost effective, strengthening or
retrofitting is an effective way to strengthen the same. The most popular techniques for strengthening of RC
beams have involved the use of external epoxy-bonded steel plates. It has been found experimentally that
flexural strength of a structural member can increase by using this technique. Although steel bonding technique
is simple, cost-effective and efficient, it suffers from a serious problem of deterioration of bond at the steel and
concrete interphase due to corrosion of steel. Other common strengthening technique involves construction of
steel jackets which is quite effective from strength, stiffness and ductility considerations. However, it increases overall cross-sectional dimensions, leading to increase in self-weight of structures and is labour intensive. To
eliminate these problems, steel plate was replaced by corrosion resistant and light-weight FRP Composite plates.
FRPCs help to increase strength and ductility without excessive increase in stiffness. Further, such material
could be designed to meet specific requirements by adjusting placement of fibres. So concrete members can now
be easily and effectively strengthened using externally bonded FRP composites. By wrapping FRP sheets,
retrofitting of concrete structures provide a more economical and technically superior alternative to the
traditional techniques in many situations because it offers high strength, low weight, corrosion resistance, high
fatigue resistance, easy and rapid installation and minimal change in structural geometry. FRP systems can also
be used in areas with limited access where traditional techniques would be impractical. However, due to lack of
the proper knowledge on structural behavior of concrete structures, the use of these materials for retrofitting the
existing concrete structures cannot reach up to the expectation. Successful retrofitting of concrete structures with FRP needs a thorough knowledge on the subject and available user-friendly technologies/ unique guidelines.
Beams are the critical structural members subjected to bending, torsion and shear in all type of
structures. Similarly, columns are also used as various important elements subjected to axial load combined
with/without bending and are used in all type of structures. Therefore, extensive research works are being
carried out throughout world on retrofitting of concrete beams and columns with externally bonded FRP
composites. Several investigators took up concrete beams and columns retrofitted with carbon fibre reinforced
polymer (CFRP)/ glass fibre reinforced polymer (GFRP) composites in order to study the enhancement of
strength and ductility, durability, effect of confinement, preparation of design guidelines and experimental
investigations of these members.
1.2 Flexural Strenghtening of Beams For flexural strengthening, there are many methods such as: section enlargement, steel plate bonding,
external post tensioning method, near-surface mounted (NSM) system and externally bonded (EB) system.
While many methods of strengthening structures are available, strengthening structures via external bonding of
advanced fibre-reinforced polymer composite (FRP) has become very popular worldwide. During the past
decade, their application in this field has been rising due to the well-known advantages of FRP composites over
other materials. Consequently, a great quantity of research, both experimental and theoretical, has been
conducted on the behaviour of FRP-strengthened reinforced concrete (RC) structures. In this regard, the
evolving technology of using carbon-bonded fibre-reinforced polymers (CFRP) for strengthening of RC beams
1.3 Advantages of FRP Some of the main advantages of FRP can be listed below:
Low weight: The FRP is much less dense and therefore lighter than the equivalent volume ofsteel. The lower
weight of FRP makes installation and handling significantly easier than steel. These properties are particularly
important when installation is done in cramped locations. Other works like works on soffits of bridges and
building floor slabs are carried out from man-access platforms rather than from full scaffolding. The use of fibre
composites does not significantly increase the weight of the structure or the dimensions of the member. And
because of their light weight, the transport of FRP materials has minimal environmental impact.
Mechanical strength: FRP can provide a maximum material stiffness to density ratio of 3.5to 5 times that of
aluminium or steel. FRP is so strong and stiff for its weight, it can out-perform the other materials.
Formability: The material can take up irregularities in the shape of the concrete surface. Itcan be moulded to almost any desired shape. We can create or copy most shapes with ease.
Chemical resistance: FRP is minimally reactive, making it ideal as a protective covering forsurfaces where
chemical
Joints: Laps and joints are not required.
Corrosion resistance: Unlike metal, FRP does not rust away and it can be used to make long-lasting structures.
Low maintenance: Once FRP is installed, it requires minimal maintenance. The materialsfibres and resins are
durable if correctly specified, and require little maintenance. If they are damaged in service, it is relatively
simple to repair them, by adding an additional layer.
Long life: It has high resistance to fatigue and has shown excellent durability over the last 50 years.
Easy to apply: The application of FRP plate or sheet material is like applying wallpaper;once it has been rolled
on carefully to remove entrapped air and excess adhesive it may be left unsupported. Fibre composite materials
are available in very long lengths while steel plate is generally limited to 6 m. These various factors in combination lead to a significantly simpler and quicker strengthening process than when using steel plate.
1.4 Suitability of Frp for Uses in Structural Engineering The strength properties of FRPs collectively make up one of the primary reasons for which civil
engineers select them in the design of structures. A material's strength is governed by its ability to sustain a load
without excessive deformation or failure. When an FRP specimen is tested in axial tension, the applied force per
unit cross-sectional area (stress) is proportional to the ratio of change in a specimen's length to its original length
(strain). When the applied load is removed, FRP returns to its original shape or length. In other words, FRP
responds linear-elastically to axial stress. The response of FRP to axial compression is reliant on the relative
proportion in volume of fibres, the properties of the fibre and resin, and the interface bond strength. FRP
composite compression failure occurs when the fibres exhibit extreme (often sudden and dramatic) lateral or sides-way deflection called fibre buckling. FRP's response to transverse tensile stress is very much dependent on
the properties of the fibre and matrix, the interaction between the fibre and matrix, and the strength of the fibre-
matrix interface. Generally, however, tensile strength in this direction is very poor. Shear stress is induced in the
plane of an area when external loads tend to cause two segments of a body to slide over one another. The shear
strength of FRP is difficult to quantify. Generally, failure will occur within the matrix material parallel to the
fibres. Among FRP's high strength properties, the most relevant features include excellent durability and
corrosion resistance. Furthermore, their high strength-to-weight ratio is of significant benefit; a member
composed of FRP can support larger live loads since its dead weight does not contribute significantly to the
loads that it must bear. Other features include ease of installation, versatility, anti-seismic behaviour,
electromagnetic neutrality, excellent fatigue behaviour, and fire resistance. However, like most structural
materials, FRPs have a few drawbacks that would create some hesitancy in civil engineers to use it for all
applications: high cost, brittle behaviour, susceptibility to deformation under long-term loads, UV degradation, photo-degradation (from exposure to light), temperature and moisture effects, lack of design codes, and most
importantly, lack of awareness.
1.5 Applications of Frp Composites in Construction There are three broad divisions into which applications of FRP in civil engineering can be classified:
applications for new construction, repair and rehabilitation applications, and architectural applications. FRPs
have been used widely by civil engineers in the design of new construction. Structures such as bridges and
columns built completely out of FRP composites have demonstrated exceptional durability, and effective
resistance to effects of environmental exposure. Pre-stressing tendons, reinforcing bars, grid reinforcement and
dowels are all examples of the many diverse applications of FRP in new structures. One of the most common
uses for FRP involves the repair and rehabilitation of damaged or deteriorating structures. Several companies across the world are beginning to wrap damaged bridge piers to prevent collapse and steel-reinforced columns to
improve the structural integrity and to prevent buckling of the reinforcement. Architects have also discovered
the many applications for which FRP can be used. These include structures such as siding/cladding, roofing,
flooring and partitions.
1.6 Current Research on FRP A serious matter relating to the use of FRPs in civil applications is the lack of design codes and
specifications. For nearly a decade now, researchers from Canada, Europe, and Japan have been collaborating
their efforts in hope of developing such documents to provide guidance for engineers designing FRP structures.
1.7 Design Considerations The development of the advanced composite technology is an engineer's dream for innovative design
and application. The characteristics of a composite can be tailored and designed to meet any desired
specifications. Most of the information and design data available on composites are in the aerospace
applications, but they are protected under the guise of proprietary systems and/or military classified documents. Unlike conventional isotropic materials of steel and concrete, there are no readily available design charts and
guidelines to help the structural engineer. When it comes to working with composites as opposed to
conventional materials, as the author has discovered, the difference can be as dramatic as night and day.
1.8 Disadvantages of Frp The main disadvantage of externally strengthening structures with fibre composite materials is the risk
of fire, vandalism or accidental damage, unless the strengthening is protected. A particular concern for bridges
over roads is the risk of soffit reinforcement being hit by over-height vehicles.
A perceived disadvantage of using FRP for strengthening is the relatively high cost of the materials. However,
comparisons should be made on the basis of the complete strengthening exercise; in certain cases the costs can
be less than that of steel plate bonding. A disadvantage in the eyes of many clients will be the lack of experience
of the techniques and suitably qualified staff to carry out the work. Finally, a significant disadvantage is the lack of accepted design standards.
II. Review Of Literature 2.1 Brief Review
This chapter provides a review of literature on strengthening of RC concrete beams. This review
comprises of literature on strengthened beam under two types of support condition i.e. simply supported and
continuously supported.
2.1.1 Simply Supported Beam Grace et al. (1999) investigated the behaviour of RC beams strengthened with CFRP and GFRP sheets
and laminates. They studied the influence of the number of layers, epoxy types, and strengthening pattern on the
response of the beams. They found that all beams experienced brittle failure, with appreciable enhancement in
strength, thus requiring a higher factor of safety in design.
Experimental investigations, theoretical calculations and numerical simulations showed that
strengthening the reinforced concrete beams with externally bonded CFRP sheets in the tension zone
considerably increased the strength at bending, reduced deflections as well as cracks width (Ross et al., 1999;
Sebastian, 2001; Smith & Teng, 2002; Yang et al., 2003; Aiello & Ombres, 2004). It also changed the behaviour
of these beams under load and failure pattern. Most often the strengthened beams failed in a brittle way, mainly
due to the loss of connection between the composite material and the concrete. The influence of the surface
preparation of the concrete, adhesive type, and concrete strength on the overall bond strength is studied as well
as characteristics of force transfer from the plate to concrete. They concluded that the surface preparation along with along with soundness of concrete could influence the ultimate bond strength. Thereafter, Study on de-
bonding problems in concrete beams externally strengthened with FRP composites are carried out by many
researchers.
Many investigators used externally bonded FRP composites to improve the flexural strength of
reinforced concrete members. To evaluate the flexural performance of the strengthened members, it is necessary
to study flexural stiffness of FRP strengthened members at different stages, such as pre-cracking, post-cracking
and post-yielding. However, only few studied are focused on the reinforced concrete members strengthened
under pre-loading or pre-cracking (Arduni & Nanni, 1997).
F. Ceroni(2010) investigated the experimental program on Reinforced Concrete (RC) beams externally
strengthened with carbon Fibre Reinforced Plastic (FRP) laminates and Near Surface Mounted (NSM) bars
under monotonic and cyclic loads, the latter ones characterized by a low number of cycles in the elastic and
post-elastic range. Comparisons between experimental and theoretical failure loads are discussed in detail. Obaidat et al. (2010) studied the Retrofitting of reinforced concrete beams using composite laminates
and the main variables considered are the internal reinforcement ratio, position of retrofitting and the length of
CFRP. The experimental tests were performed to investigate the behaviour of beams designed in such a way that
either flexural or shear failure will be expected. The beams were loaded in four-point bending until cracks developed. The beams were then unloaded and retrofitted with CFRP. Finally the beams were loaded until
failure. The ABAQUS program was used to develop finite element models for simulation of the behaviour of
beams. The concrete was modelled using a plastic damage model and two models, a perfect bond model and a
cohesive model, were evaluated for the concrete-CFRP interface. From the analyses the load-deflection
relationships until failure, failure modes and crack patterns were obtained and compared to the experimental
results. The FEM results agreed well with the experiments when using the cohesive model regarding failure
mode and load capacity while the perfect bond model was not able to represent the debonding failure mode. The
results showed that when the length of CFRP increases the load capacity of the beam increases both for shear
and flexural retrofitting. FEM results also showed that the width and stiffness of CFRP affect the failure mode
of retrofitted beams. The maximum load increases with increased width. Increased CFRP stiffness increases the
maximum load only up to a certain value of the stiffness, and thereafter it decreases the maximum load. In another research, Hee Sun Kim (2011) carried on experimental studies of 14 reinforced concrete
(RC) beams retrofitted with new hybrid fibre reinforced polymer (FRP) system consisting carbon FRP (CFRP)
and glass FRP (GFRP). The objective of this study was to examine effect of hybrid FRPs on structural behavior
of retrofitted RC beams and to investigate if different sequences of CFRP and GFRP sheets of the hybrid FRPs
have influences on improvement of strengthening RC beams. The beams are loaded with different magnitudes
prior to retrofitting in order to investigate the effect of initial loading on the flexural behavior of the retrofitted
beam. The main test variables are sequences of attaching hybrid FRP layers and magnitudes of preloads. Under
loaded condition, beams are retrofitted with two or three layers of hybrid FRPs, then the load increases until the
beams reach failure. Test results conclude that strengthening effects of hybrid FRPs on ductility and stiffness of
RC beams depend on orders of FRP layers.
2.1.2 Continuous Beam Although several research studies have been conducted on the strengthening of simply supported
reinforced concrete beams using external plates, there is very less reported work on the behaviour of
strengthened continuous beams. Moreover, most design guidelines have been developed for simply supported
beams with external FRP laminates. A critical literature review revealed that a minimum amount of research
work had been done for addressing the possibility of strengthening the negative moment region of continuous
beam using FRP materials. Grace et al., (1999) tested five continuous beams. Four different strengthening
systems were examined. The first beam was strengthened only for flexure, while the second beam was
strengthened for both flexure and shear. The third beam was strengthened with glass fibre reinforced polymer
(GFRP) sheets, and the fourth beam was strengthened by using CFRP plates. The fifth beam was fabricated as
control beam. All the beams were loaded and unloaded for at least one loading cycle before failure. The use of
FRP laminates to strengthen continuous beams was effective for reducing deflections and for increasing their load carrying capacity. It was also concluded that the beams strengthened with FRP laminates exhibit smaller
and better distributed cracks.
Grace et al., (2001) investigated the experimental performance of CFRP strips used for flexural
strengthening in the negative moment region of a full-scale reinforced concrete beam. They considered two
categories of beams (I and II) for flexural strengthening. Category I beams were designed to fail in shear and
Category II beams were designed to fail in flexure. Five full scale concrete beams of each category were tested.
It was found that Category I beams failed by diagonal cracking with local debonding at the top of the beams,
meanwhile Category II beams failed by delamination at the interface of the CFRP strips and the concrete
surface, both with and without concrete-cover failure by means shear/tension delamination. When the beams
failed, the CFRP strips were not stressed to their maximum capacity, which led to ductile failures in all the
beams. The maximum increase of load-carrying capacity due to strengthening was observed to be 29% for
Category I beams, and 40% for Category II beams with respect to corresponding control beams. On the other hand, Grace et al., (2005) performed another research work where three continuous beams
were tested. One of those beam was considered as the reference beam and conventional ductile flexural failure
occurred. They strengthened the other two beams along their negative and positive moment regions around the
top and bottom face on both sides as a U-wrap. It was concluded that the strengthened beams with the triaxial
fabric showed greater ductility than those strengthened with CFRP sheets.
In another research, El-Refaie et al., (2003) examined 11 reinforced concrete (RC) two-span beams
strengthened in flexure with external bonded CFRP sheets. According to the arrangement of the internal steel
reinforcement, the beams were classified into two groups. Each group included one non-strengthened reference
beam. It was noted that, all strengthened beams exhibited less ductility compared with the non-strengthened
control beams. An optimum number of CFRP layers were found beyond which there was no further
enhancement in the beam capacity. It was also investigated that extending the CFRP sheet length to cover the
entire hogging or sagging zones did not prevent peeling failure of the CFRP sheets, which was the dominant
failure mode of tested beams. More recently, El-Refaie et al., (2003) tested five reinforced concrete continuous beams strengthened
in flexure with external CFRP laminates. All beams had the same geometrical dimensions and internal steel
reinforcement. The main parameters examined were the position and form of the CFRP laminates. Three of the
beams were strengthened using different lay-up arrangements of CFRP reinforcement, and one was strengthened
using CFRP sheets. The performance of the CFRP strengthened beams was compared with a non-strengthened
reference beam. It was found that, peeling failure was the principal failure mode for all the strengthened tested
beams. It was found that the longitudinal elastic shear stresses at the adhesive/concrete interface calculated at
beam failure were close to the limiting value recommended in (Concrete Society Technical Report 55, 2000).
They also found that, strengthened beams at both sagging and hogging zone produced the highest load capacity.
Ashour et al., (2004) tested 16 reinforced concrete (RC) continuous beams with different arrangements
of internal steel bars and external CFRP laminates. All test specimens had the same geometrical dimensions and were classified into three groups according to the amount of internal steel reinforcement. Each group included
one non-strengthened control beam designed to fail in flexure. Three failure modes were observed, namely
laminate rupture, laminate separation and peeling failure of the concrete cover attached to the composite
laminate. The ductility of all strengthened beams was reduced in comparison with their respective reference
beam. Additionally, simplified methods for estimating the flexural load capacity and the interface shear stresses
between the adhesive and the concrete material were presented. As in previous studies, they observed that
increasing the CFRP sheet length in order to cover the entire negative or positive moment zones did not prevent
peeling failure of the CFRP laminates.
Aiello et al., (2007) compared the behaviour between continuous RC beams strengthened with of
CFRP sheets at negative or positive moment regions and RC beams strengthened at both negative and positive
moment regions. All the beams were strengthened with one CFRP sheet layer and with the remark that the
beams were not loaded at the middle of span. The control beams underwent a typical flexural and failure of the strengthened beams occurred by debonding of the CFRP sheets, together with concrete crushing. It was found
out that when the strengthening was applied to both hogging and sagging regions, the ultimate load capacity of
the beams was the highest and about 20% of moment redistribution could be achieved by CFRP sheets
externally glued in the sagging region.
Recently, Maghsoudi et al., (2009) examined the flexural behaviour and moment redistribution of
reinforced high strength concrete (RHSC) continuous beams strengthened with carbon fibre. They observed that
by increasing the number of CFRP layers, the ultimate strength increases, meanwhile ductility, moment
redistribution, and ultimate strain of CFRP sheet decrease. Test results also showed that by increasing the
number of CFRP sheet layers, there was a change in the failure mode from tensile rupture to IC debonding. End
U-straps were effective in limiting end debonding, but not intermediate span debonding.
Again, Akbarzadeh et al., (2010) conducted an experimental program to study the flexural behaviour and moment redistribution of reinforced high strength concrete (RHSC) continuous beams strengthened with
CFRP and GFRP sheets. As the previous work, test results showed that by increasing the number of CFRP sheet
layers, the ultimate strength increases, while ductility, moment redistribution, and ultimate strain of CFRP sheet
decrease. However, by using the GFRP sheets in strengthening the continuous beams, it is possible to reduce the
loss in ductility and moment redistribution but a significant increase in the ultimate strength cannot be achieved.
The moment enhancement ratio of the strengthened continuous beams was significantly higher than the ultimate
load enhancement ratio for the same beam. They also developed an analytical model for moment–curvature and
load capacity which they used for the tested continuous beams in this current study and in other similar
researches.
Finally, Majid Mohammed Ali Kadhim (2011) focused on the behavior of the high strength concrete
continuous beam strengthened with carbon fibre-reinforced polymer (CFRP) sheet with different CFRP sheet
lengths. Three full-scale continuous beams are analyzed under two points load, and the data of analysis are compared with the experimental data provided by other researchers. ANSYS program is used and the results
obtained from analysis give good agreement with experimental data with respect to load–deflection curve,
ultimate strength, and the crack patterns. The length of CFRP sheet is changed in the negative and positive
regions and the results showed that the ultimate strength of the beam was reached when the value of
Lsheet/Lspan reaches 1.0, and when the value decreases, the ultimate strength of beam also decreases a little
(1.4%), but when it decreases less than 0.6, the ultimate strength also decreases a lot (15%).
From the above information, it is, thus, clear that there lies a vast scope of research in the field of
retrofitting of RC continuous beam. Although a great deal of research has been carried out on simply supported
reinforced concrete (RC) beams strengthened with Fibre Reinforced Polymer composites (FRP), a few works
reinforcement throughout the length and two 10 mm diameter bars at top tension zone. To strengthen the beam
in shear, two different diameter bars is used for stirrups, 10 mm diameter is used in the shear zone of
intermediate support and 8mm diameter is used in the zone of end support. The diameter variation is given due to higher shear force in intermediate or continuous support than end support. Series S2 beams were reinforced
with two high-yield Strength Deformed bars of 10 mm diameter at the bottom and two 10 mm diameter bars at
top tension zone, 6 mm bars were used as hanger bars, closed stirrups of 8 mm diameter high-yield Strength
Deformed bars at 100 mm centres were provided to prevent shear failure.
Three bars of each diameter rods were tested in tensile and the measured average yield strength is averaged and
shown in Table 3.3. The modulus of elasticity of steel bars was 2 × 105 MPa.
Table 3.2 Tensile Strength of the bars
Diameter of the reinforcement Tensile strength
(mm) (MPa)
8 523
10 429
12 578
3.1.2 Detailing Of Reinforcement For the same series of continuous reinforced concrete beams, same arrangement for flexure and shear
reinforcement is made.
Figure 3.1 Detailing of reinforcement 1, 2 – top and bottom steel reinforcement
3.1.6 Curing Of Concrete Curing is done to prevent the loss of water which is essential for the process of hydration and hence for
hardening. Here curing is done by spraying water on the jute bags spread over the surface for a period of 28 days.
3.2 Strengthening Of Beams At the time of bonding of fiber, the concrete surface is made rough using a coarse sand paper texture
and then cleaned with an air blower to remove all dirt and debris. The fabrics are cut according to the size and
after that the epoxy resin is mixed in accordance with manufacturer’s instructions. The mixing is carried out in a
plastic container (100 parts by weight of Araldite LY 556 to 10 parts by weight of Hardener HY 951). After the
uniform mixing, the epoxy resin is applied to the concrete surface. Then the GFRP sheet is placed on top of
epoxy resin coating and the resin is squeezed through the roving of the fabric with the roller. Air bubbles
entrapped at the epoxy/concrete or epoxy/fabric interface are eliminated. This operation is carried out at room
temperature. Concrete beams strengthened with glass fiber fabric are cured for at least 7 days at room temperature before testing.
Figure 3.4 Application of epoxy and hardener on the beam
Figure 3.5 Roller used for the removal of air bubble
3.3 Experimental Setup The beams are tested in the loading frame of the ―Structural Engineering‖ Laboratory of National
Institute of Technology, Rourkela. The testing procedure for the all the specimen is same. The two-point loading
arrangement is used for testing of beams. Two-point loading is conveniently provided by the arrangement
shown in Figure 3.6.
The load is transmitted through a load cell and spherical seating on to a spreader beam. The spreader
beam is installed on rollers seated on steel plates bedded on the test member with cement in order to provide a
smooth leveled surface. The test member is supported on roller bearings acting on similar spreader plates. The specimen is placed over the two steel rollers bearing leaving 150 mm from the ends of the beam. The remaining
1000 mm is divided into two equal parts of 500 mm. Two dial gauges are placed just below the center of the mid
span of the beam i.e. just below the load point for recording the deflection of the beams.
3.5 Determination of Ultimate Stress, Ultimate Laod and Young’s Modulus The ultimate stress, ultimate load and young’s modulus was determined experimentally by performing
unidirectional tensile test on the specimens cut in longitudinal and transverse direction. The dimensions of the
specimens are shown in Table 3.4. The specimens were cut from the plates by diamond cutter or by hex saw.
After cutting by hex saw, it was polished in the polishing machine. For measuring the young’s modulus, the
specimen is loaded in INSTRON 1195 universal tensile test machine to failure with a recommended rate of
extension. Specimens were gripped in the upper jaw first and then gripped in the movable lower jaw. Gripping
of the specimen should be proper to prevent slippage. Here, it is taken as 50 mm from each side. Initially, the
stain is kept zero. The load as well as extension was recorded digitally with the help of the load cell and an
extensometer respectively. From these data, stress versus stain graph was plotted, the initial slope of which gives
the Young’s modulus. The ultimate stress and the ultimate load were obtained at the failure of the specimen.
The average value of each layer of the specimens is given in the Table 3.5.
Table 3.4 Result of the specimens
3.6 Testing Of Beams All the fourteen beams are tested one by one. All of them are tested in the above arrangement. The
gradual increase in load and the deformation in the dial gauge reading are taken throughout the test. The load at
which the first visible crack is developed is recorded as cracking load. Then the load is applied till the ultimate
failure of the beam. The deflections at midpoint of each span are taken for all beams with and without GFRP
and are recorded with respect to increase of load. The data furnished in this chapter have been interpreted and
discussed in the next chapter to obtain a conclusion.
STRENGHENED BEAM 5 (SB5) Same arrangement of FRP was made as SB4 and to enhance the capacity of beam SB4, two layers of complete U-wrap was provided in place of one layer and layers of U-strip of width 10 cm was applied instead of one
layer.
Figure 3.24 Cracking pattern at lower load value
Figure 3.25 Rupture of GFRP sheet at mid section of the right span
3.6.8 BEAM-8
STRENGHENED BEAM 6 (SB6) Above the U- Jacketed double Layered GFRP, more two layers of FRP but half of the width of the first two
layers, was applied at the flexural zone to prevent the flexural failure. In this case, instead of tensile rupture,
debonding failure occurred as shown in Figure 3.27.
Figure 3.26 Debonding of FRP and cracking pattern above central support of the beam
Figure 3.27 Debonding failure of Strengthened beam SB6
STRENGHENED BEAM 7 (SB7) The depth of the neutral axis was found out and the GFRP was provided up to the Neutral axis from the tension face. Here, shear crack was found and debonding occurred as shown in Figure 3.29.
Figure 3.28 Strengthening pattern of SB7
Figure 3.29 Shear crack in the left span
Figure 3.30 Magnified view of shear crack and debonding of GFRP
3.6.10 BEAM-10
STRENGHENED BEAM 8 (SB8) The no. of FRP layers was increased here as compared to SB7 to examine the changes in load capacity or the
failure pattern. The failure mode of the beam was debonding as shown in Figure 3.32.
TB1 The strengthened beam showed crack at a load of 110 KN and failed by debonding failure in which the FRP sheet was separated without concrete cover at 224 KN which is shown in Figure 3.37. The rupture of FRP sheet
was sudden and accompanied by a loud noise indicating a rapid release of energy and a total loss of load
capacity. By strengthening the RC beam using GFRP sheet, the cracking of the beam can be delayed and
flexural capacity can be increased.
Figure 3.36 Top FRP of Beam TB1 before Testing
Figure 3.37 FRP sheet separations without concrete
3.6.13 BEAM-13
TB2 Full double layered U-wrap was applied and six layers of FRP above the central support. The
ultimate failure load was 298 KN.
Figure 3.38 Experimental set up and strengthening pattern of TB2
Figure 3.39 Failure of the beam by tensile rupture
TB3 Above the U- Jacketed double Layered GFRP, more two layers of FRP but half of the width of the first two layers, was applied at the flexural crack zone to prevent the flexural failure. In this case, instead of tensile
rupture, debonding failure occurred as shown in Figure 3.41 and the failure load was 326 KN.
Figure 3.40 Strengthened beam TB3
Figure 3.41 Failure of beam TB3
Figure 3.42 Shear crack in the left span
Figure 3.43 Failure mode of TB3
IV. Test Results And Discussions The beams were loaded with a concentrated load at the middle of each span and the obtained
experimental results are presented and discussed subsequently in terms of the observed mode of failure and
load-deflection curve. The crack patterns and the mode of failure of each beam are also described in this
chapter. All the beams are tested for their ultimate strengths and it is observed that the control beam had less
load carrying capacity than the strengthened beam. Two sets of beams i.e. S1 and S2 were examined and one
beam from each series was tested as un-strengthened control beam and rest beams were strengthened with various patterns of FRP sheets. The different failure modes of the beams were observed for both the series S1
strengthened beams was higher than that of the control beams. Increasing the numbers of GFRP layers generally
reduced the mid span deflection and increased the beam stiffness for the same value of applied load. The use of
GFRP sheet had effect in delaying the growth of crack formation.
The ultimate failure load for all the tested beams are summarized in Table 4.1 and Table 4.2. The ultimate load enhancement ratio (λ), which is the ratio of the ultimate load of the externally strengthened beam to the control beam, is presented in Table 4.1 and Table 4.2. From the two tables it is found that, addition of GFRP
layers increased the ultimate load capacity and by introducing the anchoring system, the enhancement of load capacity can be done.
4.1.2.1 Strengthened Beam of S1 Series
Figure 4.1 Load versus Deflection Curve for CB1
Beam 1 was taken as the control beam (CB1) which is weak in flexure and no strengthening was done
to this beam. Two point static loading was applied on the beam and at the each increment of the load, deflection
at midpoint of each span were taken with the help of dial gauges. Using this load and deflection data, load vs.
deflection curve was plotted. At the load of 70 KN initial hairline cracks appeared. Later with the increase in loading values the crack propagated further. The Beam CB1 failed completely in flexure at the load of 260 KN.
Figure 4.2 Load versus Deflection Curve for SB1
Beam-2, SB1 is strengthened by applying GFRP at the soffit from support to support and at the top
between two load points. At the midpoint of each span, deflection values were taken and load versus deflection
curve was plotted. The deflection values are less than that of the control beam for the same load value. At the
load of 110 KN initial hairline cracks appeared. Later with the increase in loading values the crack propagated
further. At lower load, debonding of FRP without concrete cover occurred and SB1 finally failed in concrete
Beam-3, SB2 is strengthened with U-wrap from support to support distance and at the top of the beam
between the two load points. The deflection values are less than that of the control beam for the same load
value. No initial hairline cracks were visible due to the covering of GFRP. Later with the increase in loading values the crack propagated further under the GFRP. Tensile rupture took place at lower load and as the load
increased, debonding of the FRP occurred with concrete cover and finally the beam failed in shear and the
failure load was 325 KN.
Figure 4.4 Load versus Deflection Curve for SB3
Beam-4, SB3 is strengthened with U-wrap from support to support distance, but the layers were
increased and at the top of the beam between the two load points. The beam failed in debonding of FRP without
concrete cover. The deflection values are remarkably less than that of the control beam and beam SB1 for the
same load value. The cracking load was 120 KN and the failure load was 334 KN.
Figure 4.5 Load versus Deflection Curve for SB4
Beam-5, SB4 is strengthened providing FRP at the soffit of the beam from support to support distance
and U-wrap above it, and at the top of the beam between the two load points and U-strip above it. Tensile
rupture of FRP without concrete cover occurred and later with the increase in loading values the crack
propagated further under the GFRP and beam failed in flexure. The failure load of SB4 was 370 KN. The
deflection values are again remarkably less than that of the control beam for the same load value.
Figure 4.6 Load versus Deflection Curve for SB5
Beam-6, SB5 is strengthened providing FRP at the soffit of the beam from support to support distance
and U-wrap above it, and at the top of the beam between the two load points and U-strip above it. Here the
numbers of FRP layers of U-wrap and U-strip were increased. Tensile rupture of FRP without concrete cover
occurred at lower load value and later with the increase in loading values the crack propagated further under the
GFRP and beam failed in flexure. The deflection values are less than that of the control beam for the same load value. The failure load of SB5 was 380 KN. The ultimate load of this beam was higher than the beam SB4,
which was having same pattern of FRP wrapping.
Figure 4.7 Load versus Deflection Curve for SB6
Beam-7, SB6 is strengthened providing U-wrap FRP from support to support distance and U-wrap FRP
of half of the width above it, and at the top of the beam between the two load points. Debonding of FRP without
concrete cover occurred first and later with the increase in loading values the crack propagated further under the
GFRP and beam failed in flexure. The deflection values are quite less than that of the control beam for the same
load value. The failure load of SB6 was 415 KN.
Figure 4.8 Load versus Deflection Curve for SB7
Beam-8, SB7 is strengthened providing U-wrap FRP from support to support distance up to Neutral
axis and U-wrap FRP at the top of the beam between the two load points up to Neutral axis. Debonding of FRP
without concrete cover occurred, with the increase in loading values the shear crack developed and propagated
and beam failed in shear. The deflection values are quite less than that of the control beam for the same load
value. The failure load of SB7 was 332 KN.
Figure 4.9 Load versus Deflection Curve for SB8
Beam-9, SB8 is strengthened providing U-wrap FRP from support to support distance up to Neutral
axis and U-wrap FRP at the top of the beam between the two load points up to Neutral axis. Here the layers of
the U-wrap were increased. Beam failed in debonding of FRP without concrete cover. Here also, the deflection
values are quite less than that of the control beam for the same load value. The failure load of SB8 was 345 KN.
Figure 4.10 Load versus Deflection Curve for SB9
Beam-10, SB9 is strengthened as beam SB6, i.e. U-wrap FRP from support to support distance and U-wrap FRP of half of the width above it, and at the top of the beam between the two load points. Here, to prevent
debonding failure anchoring system was introduced. It took more load than the corresponding beam SB6 and up
to some load values it prevented the debonding failure. It prevented the debonding failure up to some extent and
finally failed in flexure. The deflection values are quite less than that of the control beam for the same load
provided to strengthen the beams. The midpoint deflection was compared with the control beam and shown in
Figure 4.13.
Figure 4.14 Load versus Deflection Curve for CB1, SB7, SB8
In SB7, two and four layers of U-wrap GFRP were provided below and above the Neutral axis
respectively and in case of SB8 the GFRP layers were increased to three and six respectively. The midpoint
deflections of SB1 and SB8 were compared to CB1 and from the plotted graphs and it is concluded that, by
increasing the GFRP layers the stiffness of the beam can be increased.
Figure 4.15 Load versus Deflection Curve for CB1, SB6, SB9
In SB9, Steel bolts were used to prevent the debonding failure of FRP. Here, the load capacity of SB9 was higher than SB6, the deflection values were less than CB1 as shown in Figure 4.15.
Figure 4.16 Ultimate Load Capacity of Series S1 beams
Figure 4.17 Percentage increase in the Ultimate Load Carrying capacity of strengthened beams of S1 w.r.t CB1
From Figure 4.16, it is concluded that the load capacity of SB9 beam is highest and SB6 beam has
second highest load capacity among all the strengthened beams of Series S1. The percentage increase of load
capacity of all the beams are calculated and are drawn in Figure 4.17 from which it can be concluded that, by application of GFRP to the beams the load capacity can be enhanced. Strengthened beam SB6 and SB9 gives the
maximum percentage increase of load capacity.
4.1.2.2 Strengthened Beam of S2 Series
Figure 4.18 Load versus Deflection Curve for CB2
Beam 11, Control Beam for set S2, CB2, to which no external strengthening was provided, two point
static loading was applied and at the each increment of the load, deflections at midpoint of each span were taken
with the help of dial gauges. Using this load and deflection data, load vs. deflection curve was plotted. At the
load of 110 KN initial hairline cracks appeared and the beam failed in flexure with an ultimate load value of 200
KN.
Figure 4.19 Load versus Deflection Curve for TB1
Beam-12, TB1 is strengthened at the soffit from support to support and at the top between two load
points. At the midpoint of each span, deflection values were taken and load versus deflection curve was plotted.
The deflection values are less than that of the control beam for the same load value. At lower load value, debonding of FRP without concrete cover occurred and TB1 finally failed in concrete crushing. At the load of
120 KN initial hairline cracks appeared. Later with the increase in loading values the cracks propagated further
and the beam failed with an ultimate load of 224 KN.
The load capacity and the percentage increase of all the strengthened beams of series S2 are discussed
here and from Figure 4.23 and Figure 4.24, it is found that beam TB3 has the maximum load capacity and
maximum percentage increase of load carrying capacity respectively.
V. Finite Element Analysis Finite element method (FEM) is a numerical method for solving a differential or integral equation. It
has been applied to a number of physical problems, where the governing differential equations are available.
The method essentially consists of assuming the piecewise continuous function for the solution and obtaining
the parameters of the functions in a manner that reduces the error in the solution.
5.1 Formulation The governing equation for beam is given in Equation 5.1.
M =
d2 y
EI (5.1)
d x2
The displacement field v(x) assumed for the beam element should be such that it takes on the values of
deflection and the slope at either end as given by the nodal values vi,ᶿ i,vj,ᶿ j.
The v(x) can be given by,
v(x) = c0 + c1x + c2 x2 +c3x3 (5.2)
In solving the differential equations through integration, there will be constants of integration that must be
evaluated by using the boundary and continuity conditions. The variables whose values are to be determined are
approximated by piecewise continuous polynomials. The coefficients of these polynomials are obtained by
minimizing the total potential energy of the system. In FEM, usually, these coefficients are expressed in terms
of unknown values of primary variables. Thus, if an element has got n nodes, the displacement field u can be approximated as,
u =∑n Ni ui (5.3)
i=1
where ui are the nodal displacements in x-direction and Ni are the shape functions, which are functions of
coordinates. Shape functions or interpolation functions Ni are used in the finite element analysis to interpolate
the nodal displacements of any element to any point within each element. The beam element has modulus of elasticity E, moment of inertia I, and length L. Each beam element has two nodes and is assumed to be
horizontal as shown in Figure 5.1. The element stiffness matrix is given by the following matrix, assuming axial
deformation is neglected.
12 6L −12 6L
K =
EI 6L 4L2 − 6L2L2
3
− 6L
(5.4)
L −12 12 − 6L
6L 2L2 − 6L 4L2
It is clear that the beam element has four degrees of freedom: two at each node (a transverse displacement and a
rotation). The sign convention used is that the displacement is positive if it points upwards and the rotation is
positive if it is counter clockwise. Consequently for a structure with n nodes, the global stiffness matrix K will
be of size 2 n × 2 n (since we have two degrees of freedom at each node). Once the global stiffness matrix K is
obtained we have the following structure equation
[K ] {U}={F} (5.5) where U is the global nodal displacement vector and F is the global nodal force vector.
First the boundary conditions are applied manually to the vectors U and F. Then the matrix (5.5) is solved by
partitioning and Gaussian elimination. Finally once the unknown displacements and reactions are found, the
nodal force vector is obtained for each element as follows:
Figure 5.4 Comparison of Experimental value with Numerical and Exact analysis for CB1
The numerical and experimental results for the beam are shown in Figure 5.4. The trend of the loads
varying with the deflection presents that the linear elastic state exits in the structure, when the loads are
equivalent to about 90 KN.
VI. Conclusions 6.1 Conclusions
The present experimental study is carried out on the flexural behavior of reinforced concrete
rectangular beams strengthened by GFRP sheets. Fourteen reinforced concrete (RC) beams weak in flexure having different set of reinforcement detailing are casted and tested. The beams were grouped into two series
labeled S1 and S2. Each series had different longitudinal and transverse steel reinforcement ratios. From the test
results and calculated strength values, the following conclusions are drawn:
1. The ultimate load carrying capacity of all the strengthen beams is higher when compared to the control
beam.
2. The initial cracks in the strengthened beams are formed at higher load compared to control beam.
3. From series S1, beam SB9 which was strengthened by U-wrap and was anchored by using steel plate and
bolt system, showed the highest ultimate load value of 415 KN. The percentage increase of the load
capacity of SB9 was 61.92 %.
4. The load carrying capacity of beam SB6, which was strengthened by two layers of U-wrap of length 88 cm
in positive moment zone and two layers of U-wrap of length 44 cm over first two layers, was 415 KN
which was nearer to the load capacity of beam SB9. The percentage increase of load carrying capacity was 59.61 % , from which it can be concluded that applying FRP in the flexure zone is quite effective method to
enhance the load carrying capacity.
5. TB3 beam from Series S2, which was strengthened by two layers of U-wrap in positive moment zone and
two layers of U-wrap in flexure zone above first two layers, was having maximum ultimate load value of
326 KN, than the other strengthened beams of same category. The percentage increase of this beam was 63
% which was highest among all strengthened beams.
6. Using of steel bolt and plate system is an effective method of anchoring the FRP sheet to prevent the
debonding failure.
7. Strengthening of continuous beam by providing U-wrap of FRP sheet is a new and effective way of
enhancing the capacity of load carrying.
8. Flexural failure at the intermediate support section can be prevented by application of GFRP sheets. 9. In lower range of load values the deflection obtained using Finite Element models are in good agreement
with the experimental results. For higher load values there is a deviation with the experimental results
because linear FEM has been adopted.
6.2 Scope Of The Future Work
It promises a great scope for future studies. Following areas are considered for future research:
a. Experimental study of continuous beams with opening
b. Non linear analysis of RC continuous beam
c. FEM modeling of unanchored U-wrap
d. FEM modeling of anchored U-wrap
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